What is the difference in these errors?

In a code I am writing, I keep getting the error:

In an assignment  A(:) = B, the number of elements in A and B must be the same.

And I can not find anyone else who has this error. The one I can find is:

In an assignment  A(I) = B, the number of elements in A and B must be the same.

Could someone please explain the difference in these errors to me?

Answer 1

[James Tursa]

E.g., one way to get these errors:

>> A = rand(3)
A =
    0.9649    0.9572    0.1419
    0.1576    0.4854    0.4218
    0.9706    0.8003    0.9157
>> I = A<0.5
I =
     0     0     1
     1     1     1
     0     0     0
>> A(:) = [1 2 3]
In an assignment  A(:) = B, the number of elements in A and B must be the same. 
>> A(I) = [1 2 3]
In an assignment  A(I) = B, the number of elements in B and I must be the same.

In one case, you are attempting to assign to matrix A using the equivalent of linear indexing and the number of elements doesn't match.

In the other case, you are using a logical matrix to indicate the element positions to replace, and again the number of elements doesn't match.